diff --git a/src/main/java/de/thetaphi/forbiddenapis/gradle/CheckForbiddenApis.java b/src/main/java/de/thetaphi/forbiddenapis/gradle/CheckForbiddenApis.java
index f8c4bdd3..c0970ade 100644
--- a/src/main/java/de/thetaphi/forbiddenapis/gradle/CheckForbiddenApis.java
+++ b/src/main/java/de/thetaphi/forbiddenapis/gradle/CheckForbiddenApis.java
@@ -27,6 +27,7 @@
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.EnumSet;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Set;
@@ -43,7 +44,7 @@
 import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.InputFiles;
 import org.gradle.api.tasks.Optional;
-import org.gradle.api.tasks.OutputDirectory;
+import org.gradle.api.tasks.OutputDirectories;
 import org.gradle.api.tasks.ParallelizableTask;
 import org.gradle.api.tasks.SkipWhenEmpty;
 import org.gradle.api.tasks.TaskAction;
@@ -109,23 +110,49 @@ public class CheckForbiddenApis extends DefaultTask implements PatternFilterable
   
   private final CheckForbiddenApisExtension data = new CheckForbiddenApisExtension();
   private final PatternSet patternSet = new PatternSet().include("**/*.class");
-  private File classesDir;
+  private FileCollection classesDirs;
   private FileCollection classpath;
   private String targetCompatibility;
   
+  /**
+   * Directories with the class files to check.
+   * Defaults to current sourseSet's output directory (Gradle 2/3) or output directories (Gradle 4.0+).
+   */
+  @OutputDirectories
+  // no @InputDirectories, we use separate getter for a list of all input files
+  public FileCollection getClassesDirs() {
+    return classesDirs;
+  }
+
+  /** @see #getClassesDirs */
+  public void setClassesDirs(FileCollection classesDirs) {
+    if (classesDirs == null) throw new NullPointerException("classesDirs");
+    this.classesDirs = classesDirs;
+  }
+
   /**
    * Directory with the class files to check.
-   * Defaults to current sourseSet's output directory.
+   * Defaults to current sourseSet's output directory (Gradle 2/3 only).
+   * @deprecated use {@link #getClassesDirs()} instead. If there are more than one
+   *  {@code classesDir} set by {@link #setClassesDirs(FileCollection)}, this getter may
+   *  throw an exception!
    */
-  @OutputDirectory
-  // no @InputDirectory, we use separate getter for a list of all input files
+  @Deprecated
   public File getClassesDir() {
-    return classesDir;
+    final FileCollection col = getClassesDirs();
+    return (col == null) ? null : col.getSingleFile();
   }
 
-  /** @see #getClassesDir */
+  /** Sets the directory where to look for classes. Overwrites any value set by {@link #setClassesDirs(FileCollection)}!
+   * @deprecated use {@link #setClassesDirs(FileCollection)} instead.
+   * @see #getClassesDir
+   */
+  @Deprecated
   public void setClassesDir(File classesDir) {
-    this.classesDir = classesDir;
+    if (classesDir == null) throw new NullPointerException("classesDir");
+    getLogger().warn("The 'classesDir' property on the '{}' task is deprecated. Use 'classesDirs' of type FileCollection instead!",
+        getName());
+    setClassesDirs(getProject().files(classesDir));
   }
 
   /** Returns the pattern set to match against class files in {@link #getClassesDir()}. */
@@ -149,6 +176,7 @@ public FileCollection getClasspath() {
 
   /** @see #getClasspath */
   public void setClasspath(FileCollection classpath) {
+    if (classpath == null) throw new NullPointerException("classpath");
     this.classpath = classpath;
   }
 
@@ -453,16 +481,16 @@ public CheckForbiddenApis include(@SuppressWarnings("rawtypes") Closure arg0) {
   @InputFiles
   @SkipWhenEmpty
   public FileTree getClassFiles() {
-    return getProject().files(getClassesDir()).getAsFileTree().matching(getPatternSet());
+    return getClassesDirs().getAsFileTree().matching(getPatternSet());
   }
 
   /** Executes the forbidden apis task. */
   @TaskAction
   public void checkForbidden() throws ForbiddenApiException {
-    final File classesDir = getClassesDir();
+    final FileCollection classesDirs = getClassesDirs();
     final FileCollection classpath = getClasspath();
-    if (classesDir == null || classpath == null) {
-      throw new InvalidUserDataException("Missing 'classesDir' or 'classpath' property.");
+    if (classesDirs == null || classpath == null) {
+      throw new InvalidUserDataException("Missing 'classesDirs' or 'classpath' property.");
     }
     
     final Logger log = new Logger() {
@@ -482,14 +510,15 @@ public void info(String msg) {
       }
     };
     
-    final Set<File> cpElements = classpath.getFiles();
-    final URL[] urls = new URL[cpElements.size() + 1];
+    final Set<File> cpElements = new LinkedHashSet<File>();
+    cpElements.addAll(classpath.getFiles());
+    cpElements.addAll(classesDirs.getFiles());
+    final URL[] urls = new URL[cpElements.size()];
     try {
       int i = 0;
       for (final File cpElement : cpElements) {
         urls[i++] = cpElement.toURI().toURL();
       }
-      urls[i++] = classesDir.toURI().toURL();
       assert i == urls.length;
     } catch (MalformedURLException mfue) {
       throw new InvalidUserDataException("Failed to build classpath URLs.", mfue);
diff --git a/src/main/resources/de/thetaphi/forbiddenapis/gradle/plugin-init.groovy b/src/main/resources/de/thetaphi/forbiddenapis/gradle/plugin-init.groovy
index 2f575575..a5fdd250 100644
--- a/src/main/resources/de/thetaphi/forbiddenapis/gradle/plugin-init.groovy
+++ b/src/main/resources/de/thetaphi/forbiddenapis/gradle/plugin-init.groovy
@@ -45,19 +45,20 @@ def extensionProps = CheckForbiddenApisExtension.class.declaredFields.findAll{ f
 
 // Define our tasks (one for each SourceSet):
 def forbiddenTasks = project.sourceSets.collect{ sourceSet ->
+  def getClassesDirs = { sourceSet.output.hasProperty('classesDirs') ? sourceSet.output.classesDirs : project.files(sourceSet.output.classesDir) }
   project.tasks.create(sourceSet.getTaskName(FORBIDDEN_APIS_TASK_NAME, null), CheckForbiddenApis.class) {
     description = "Runs forbidden-apis checks on '${sourceSet.name}' classes.";
     conventionMapping.with{
       extensionProps.each{ key ->
         map(key, { extension[key] });
       }
-      classesDir = { sourceSet.output.classesDir }
+      classesDirs = { getClassesDirs() }
       classpath = { sourceSet.compileClasspath }
       targetCompatibility = { project.targetCompatibility?.toString() }
     }
-    // add dependency to compile task after evaluation, if the classesDir is from our SourceSet:
-    project.afterEvaluate{
-      if (classesDir == sourceSet.output.classesDir) {
+    // add dependency to compile task after evaluation, if the classesDirs collection has overlaps with our SourceSet:
+    project.afterEvaluate{ 
+      if (!classesDirs.minus(getClassesDirs()).isEmpty()) {
         dependsOn(sourceSet.output);
       }
     }
diff --git a/src/test/gradle/build.gradle b/src/test/gradle/build.gradle
index 397e526f..41e0bd7c 100644
--- a/src/test/gradle/build.gradle
+++ b/src/test/gradle/build.gradle
@@ -37,12 +37,13 @@ forbiddenApis {
 }
 
 forbiddenApisMain {
-  classesDir = new File(forbiddenRootDir, 'build/main')
+  classesDirs = files(new File(forbiddenRootDir, 'build/main'))
   classpath = files(forbiddenClasspath.tokenize(File.pathSeparator))
   bundledSignatures += 'jdk-system-out'
 }
 
 forbiddenApisTest {
+  // use classesDir here to check backwards compatibility!:
   classesDir = new File(forbiddenRootDir, 'build/test')
   classpath = files(forbiddenTestClasspath.tokenize(File.pathSeparator))
 }